Although gene therapy and specifically human gene therapy has been widely discussed only over the last five years, the basic idea first became a reality in 1944 when Avery et al. carried out research on the chemical nature of substances inducing transformation of pneumococcal types. (Avery et al., J. Exp. Med. 79:137-158, 1944). The work carried out by Avery et al., did not involve the actual insertion of genetic material into cells in order to carry out gene therapy. The insertion of new genetic material into cells in order to permanently affect the genetic makeup of the cells is the methodology now generally referred to as gene therapy.
Current gene therapy is carried out in a variety of ways but involves two general protocols. In the first method, referred to as ex vivo gene therapy, cells are extracted from an organism such as a human and subsequently subjected to genetic manipulation by a variety of different means. After genetic material has been properly inserted into the cells, the cells are implanted back into the body from which they were removed. Thus the process involves cell removal, transformation of the cells in vitro, and subsequent reintroduction of the modified cells into the patient. Persistent, in vivo expression of the newly implanted genetic material after transplantation of the transformed cells has been successful (see Morgan et al., Science 237:1476 (1987); and Gerrard et al., Nat. Genet. 3:180 (1993)).
In the second approach to gene therapy, referred to as in vivo gene therapy, somatic cells within a living organism are transformed with new genetic material. For example, the genetic material to be introduced into the organism is packaged within a retrovirus or adenovirus. The virus containing the desired genetic material is allowed to infect target cells within the organism. Upon infection of the cells, the virus injects genetic material into the cells which is then integrated into the cells' genome. As a result, the injected genetic material is expressed and the patient is treated.
Several different methods for transforming cells can be used in accordance with either the ex vivo or in vivo transfection procedures. For example, various mechanical methods can be used to deliver the genetic material, including the use of fusogenic lipid vesicles (liposomes incorporating cationic lipids such as lipofection; see Felgner et al., Proc. Natl. Acad. Sci. U.S.A. 84:7413-7417 (1987)); direct injection of DNA (Wolff, et al., Science (1990) 247:1465-1468); and pneumatic delivery of DNA-coated gold particles with a device referred to as the gene gun (Yang et al., Proc. Natl. Acad. Sci. U.S.A. 1990; 87:1568-9572).
Ex vivo and in vivo gene transfer methodologies have been accomplished using a variety of different procedures, such as the use of retroviruses or direct injection. The procedures have been used on five general types of cells in order to carry out (1) liver cell gene therapy; (2) hematopoietic cell gene therapy; (3) cancer cell gene therapy; (4) respiratory cell gene therapy; and (5) muscle cell gene therapy. A review of the different techniques along with a citation of numerous publications in each area is contained within a recent article on human gene therapy (see Morsy et al., JAMA 270:2338-2345 (1993)).
Depending on the desired result, the effect which the inserted genetic material will have on the transformed cell can vary greatly and can be selected according to the specific therapeutic situation. For example, genetic material inserted into the cells in order to obtain circulation of the expressed genetic products would not be used in connection with the treatment of cancer cells of a localized tumor. Stated differently, gene therapy may be carried out in order to locally affect a given type of cells such as affecting cancer cells within a tumor or locally affecting liver cells. Other types of gene therapy are carried out in order to cause the manipulated cells to express a protein which is transported to the circulatory system and systemically delivered to the organism. Genetic manipulation of cells to express a protein for systemic delivery to the organism has been problematic. The present invention addresses this problem.